Even though Zdr (differential reflectivity) was first introduced to the radar meteorology community more than 40 years ago by Seliga and Bringi (1976), the calibration of Zdr continues to be a topic of research, an issue for most radars, and a quantity whose temporal stability is poorly documented and inadequately understood. In this talk temporal Zdr bias variations are investigated using the NCAR's S-band polarimetric radar (S-Pol) solar scan data collected over extended time periods. The measurements reveal a high linear correlation between the ambient temperature at the radar and Zdr. Using the solar measurements and the ratio of crosspolar powers, the components of the radar that cause the variation of the Zdr bias are identified. It is shown that the thermal expansion of the antenna is the primary cause of the Zdr bias variation. The crosspolar power (CP) technique is used for calibration of Zdr for PECAN (Plains Elevated Convection at Night) data. A technique to mitigate the Zdr bias caused by temperature variations is given. The Zdr bias from the CP technique is compared to vertical pointing Zdr bias calculations and the uncertainty of the Zdr bias estimates are given. It is also shown how the CP technique can be applied to simultaneous H and V transmit radars.